Rocket mass heater

A rocket mass heater is a variation on the rocket stove that is intended to heat a house. (It is a rather extreme variation, and there is no need to understand what a rocket stove is in order to understand the mass heater). One aspect of the rocket mass heater that you should be aware of; they are most effective in a cob or 'poured adobe' type superstructure. You can build a rocket mass heater on your own, but you will need to dedicate some real time and thought to the thermal mass aspect.

First, some definitions:

Thermal mass heater: A thermal mass heater is a type of stove, generally wood burning, that is designed to put nearly all of the heat from a fire into your building. This is a rare thing indeed, and you don't want all of that heat entering into your house at one time. Instead, the heat enters some sort of thermal mass, usually masonry. Hence, many of these are called masonry heaters. They are very fuel efficient, but they take a longer time to heat a building (because the heat goes through so much masonry/cob before it reaches you); on the up side, a soft, comfortable warmth will continue seeping out for hours after the fire has stopped. One constant design feature of thermal mass heaters is long and/or labyrinthine chimneys.

Cob: While it is not necessary to use cob to insulate your rocket mass heater, it is the best material. Cob is a mixture of clay, sand, straw, and water. This is basically the same as adobe, but while adobe is generally made into bricks, cob is added to the building one handful at a time, and allowed to dry in place. This means that you can build a brick-solid wall into any shape you want. Cob lends itself to curves, and is a great material for making benches out of. It also lends itself to building around stove pipes and 55 gallon drums, which as you will see is important. One caveat: building in cob isn't hard, but it is labor intensive.

And now, to the crux of the matter:

A rocket mass heater is a wood burning stove that burns fuel completely and cleanly. This means that you use much less wood, about 1/6 as much as you would otherwise, and you will not be risking asthma and emphysema every time you fire up your stove. The stove and the long and twisty chimney are all embedded in cob. Because you are building it yourself, you can heat your house in the way you deem most convenient. You decide how much of the stove is embedded (and thus how much heat you get immediately vs. store for slow release) and where the chimney goes (often through a cob bench or bed - comfy!).

This type of stove was invented by Ianto Evans, who markets it himself. While it is a popular and effective heater, it is hard to find details on-line, and I suspect that this is because Ianto does not want too many details leaked; you're supposed to buy the book! So I'm going to be a little sketchy on the exact details of construction, out of respect for the inventor. I will tell you that the basic design of the rocket heater is a large combustion chamber made from (usually) a 55 gallon drum with a smaller drum inside it, a smaller feed tube through which you put the fuel, and a long chimney embedded in the cob which allows for as much heat as possible to be released into the cob before venting to the outside. The stove is essentially a clever mixture of a rocket stove and a traditional European masonry heater. If you plan on building one, it is well worth buying the instruction book, not only for the design, but for Ianto's thorough understanding of problems you might encounter when building any stove of this type.

Because cob is heavy, it may not be practical to add a rocket thermal mass heater onto an existing house. You will need a strong foundation and enough space to put in a sizable stove and 20-30 feet of stovepipe embedded in mud. (The stove pipe can double back on itself within the cob, and can be horizontal for almost its entire length. The length improves its draw so much that it can afford to twist and turn.)

Another potential problem with this types of stove is that the wood you burn needs to have a diameter no greater than 3-5 inches. This can be good if you have a lot of sticks and branches to burn, but if you are burning logs you will need to do a lot of splitting.

Pluses to the rocket mass heater include cost, fuel efficiency, and sustainability. If you are a seasoned scavenger, you might well be able to get all the components for free, and save them form cluttering up the landfill. Old metal drums, bricks, and stovepipe are your basic materials, and the thermal mass can include urbanite or rocks to help save on cob.

As you might have noticed, I have been switching back and forth between calling this a stove and a heater. I should clarify that its main function is as a heater. It can be used to boil water or even cook food, but is designed to heat your house.